1. Field of the Invention
The present invention relates generally to receivers for receiving repeated transmissions of a multilevel signal, and more specifically to a method of making a symbol decision on the received multilevel signal.
2. Description of the Related Art
In conventional radio paging systems, a paging signal is repeatedly transmitted to increase the probability of successful reception. At paging receivers, signals of repeated transmissions are compared with each other on a symbol-by-symbol basis, and the compared symbols are time-diversity combined. In a known diversity method, a sum of symbol values is taken and the sum is divided by the number of symbols to produce an average value. The average value is used as a threshold for making a symbol decision. Another approach involves measuring the distance from each symbol to a reference value and a symbol nearest to the reference is adopted as a representative of the symbols.
However, the averaging method is not suitable for noisy channels because of the susceptibility of its decision thresholds to impulse noise. In the case of the nearest distance approach, a random occurrence of a false optimal (nearest) symbol invalidates many of true sub-optimal symbols. This is particularly disadvantageous when multilevel signals are transmitted.